1. Field of the Invention
The present invention relates to a new plating method and a plating bath precursor to be used therefor.
2. Description of the Related Art
A wet plating method for reducing metal ions in a bath and depositing the ions onto the surface of an object to be plated is classified roughly into an electroplating (electrolyzing deposition) method and an electroless plating (chemical deposition) method on the basis of the reduction mechanism as generally known. Both methods have merits and demerits.
For example, the electroplating method has advantages whereby, during plating, metal ions of basically the same amount as that of the metal deposited on the surface of the object to be plated are supplied from the anode, and the composition of a plating bath is maintained roughly constant, and therefore, said plating bath can be continuously used over a long period of time, however, it also has the following problems:
The object to be plated is limited to an object at least whose surface is electrically conductive.
Depending on the form of the object to be plated, since an electric charge is particularly easily concentrated onto a convex portion thereon, the thickness of the plated layer easily becomes uneven.
On the other hand, the electroless plating method has advantages whereby, the material of the object to be plated is not basically material of the restricted, and also, regardless of the form of the object to be plated, the thickness of the plated layer can be made even, however, it also has following problems:
Depending on the material of the plating metal and the object to be plated, catalysis processing by means of a palladium compound is necessary, and the production cost is high.
Since a reducing agent used for reduction of metal ions accumulates in the bath as oxidized form, and since unnecessary components inevitably contain in the plating bath by supplying a new reducing agent and metal ions to maintain the plating bath which was consumed during the plating, the composition and concentration of the bath easily change, whereby the life of the plating bath is limited.
Since the electroless plating is a metal deposition method using self-catalysis, deposition of a catalyst-poisonous metal is difficult, whereby metal types which can be used for plating are limited.
Therefore, in order to solve the above problems in the prior-art electroless plating method, Warwick et al proposed a new electroless plating method (called a xe2x80x9credox system electroless plating methodxe2x80x9d for distinction from the prior-art conventional electroless plating method), wherein, by oxidizing first metal ions of a redox system of a plating bath from a lower oxidation state to a high oxidation state, and second metal ions of said redox system are reduced and deposited onto the surface of an object to be plated (M. E. Warwick and B. Shirley; The Autocatalitic Deposition of Tin, Trans. Inst. Metal Finishing, 58, 9(1980)).
That is, in the above document, Warwick et al presented that, when Ti3+ in a plating bath was oxidized to Ti4+ (or TiO2+ in a real existing form), by using a phenomenon in that Sn2+ ions existing in the same bath were reduced to metal tin, tin autocatalytic electroless deposition which had been considered impossible by the prior-art electroless plating method was achieved, whereby they took the initiative of a redox system electroless plating method.
Thereafter, many researchers have studied the application of this redox system electroless plating method to various metal plating.
For example, in Japanese Laid-open Patent Publication No. 125379 of 1985, a gold electroless-plating bath using Ti3+ as a reducing agent is disclosed.
Also, in Japanese Laid-open Patent Publication No. 191070 of 1991, nickel, zinc, silver, cadmium, indium, antimony, and lead electroless plating bath using TiCl3 as a reducing agent are disclosed, and in Japanese Laid-open Patent Publication No. 325688 of 1992, the abovementioned various metal electroless plating bath using trivalent titanium chloride in place of TiCl3 are disclosed.
Also, in Japanese Laid-open Patent Publication No. 101056 of 1994, a tin-lead alloy using Ti3+ as a reducing agent, that is, an electroless plating bath for solder is disclosed.
Also, in Japanese Laid-open Patent Publication No. 264248 of 1994, a description is given in that, in the abovementioned redox system electroless plating method, a carbonate such as sodium carbonate or potassium carbonate is used in place of ammonia which is normally used for adjusting pH of the plating bath.
Furthermore, in Japanese Laid-open Patent publication No. 340979 of 1994, a copper plating bath, which contains thiourea as a complex forming agent of metal ions, and uses Ti3+ as a reducing agent, is disclosed, and it has been reported that this copper can be deposited even by using Co2+ in place of Ti3+ as a reducing agent (pages 33-34 of xe2x80x9cSummary report of 98th Conferencexe2x80x9d, Surface Technology Society, 1998, Seiichiro Nakao, Hidemi Nawafune, Shozo Mizumoto, Yoshiki Murakami, and Shin Hashimoto)
As mentioned above, the redox system electroless plating method has the following advantages as in the prior-art conventional electroless plating method:
basically, the material of an object to be plated is not limited, and
the plated layer can be made even in thickness regardless of the form of the object to be plated,
and further, has the following additional advantages:
as well as various metals which can be used for plating in the prior-art electroless plating method, as mentioned above, while catalyst-poisonous metals such as tin, lead, and antimony which cannot be used for the autocatalytic electroless plating in the prior-art it is possible to use them with electroless plating,
since the speed of the oxidation and reduction reaction in the redox system is faster than that of the reduction reaction of the metal ions by a reducing agent in the prior-art of electroless plating method, element such as phosphorous and boron contained in a reducing agent are co-deposited in the plated layer and there is a possibility that a plated layer can be formed more efficiently and faster than in the prior-art,
in the prior-art of electroless plating method, element such as phosphorus and boron contained in a reducing agent are co-deposited in the plated layer, and this may influence electrical, mechanical, or chemical properties of the plated layer, however, in the redox system electroless plating method, since the reducing agent containing these elements is not used, a plated layer which is made from a pure metal without containing co-deposits, is excellent in that the above properties can be formed,
and therefore, for various fields in which the electroless plating method has not been employable for forming a plated layer due to the abovementioned co-deposits, the possibility of using a redox system electroless plating method can be used arises.
However, in actuality, the redox system electroless plating method is not widely used in industry although it has many advantages as mentioned above.
The reason for this is that activity of the redox system reaction is extremely high.
That is, a redox system plating bath is unstable since it is high in activity of the system reaction as mentioned above, suspended deposition easily occurs, and when such deposition occurs, an even plated layer may not be formed.
Also, the redox system plating bath initially has a fast reaction speed since it has high activity as mentioned above, and this is advantageous in one aspect of the redox system electroless plating method as mentioned above, however, a new problem is caused whereby the life of the plating bath is shortened.
As for the former problem concerning the stability of the plating bath, for example, by examination of a complex forming agent conducted by Obata among the present inventors together with other researchers (Japanese Laid-open Patent Publication No. 185759 of 1985), some worthy results have been obtained.
However, as for the latter problem concerning the shorter life of the plating bath, an essential solution has at present not been found.
That is, at a point in time a plating bath to be used in the redox system electroless plating method is made up by adding components, oxidation of metal ions composing the redox system and the reduction of metal ions forming the plated layer is started. Whether or not an object to be plated is dipped in the bath, oxidation and reduction proceed rapidly. The rate of progression is extremely fast in comparison with that of a reduction in metal ions by a reducing agent in the prior-art conventional electroless plating method.
Moreover, in the metal ions composing the redox system, some ions do not contribute to a reduction in metal ions to form a plated layer, but are oxidized by dissolved oxygen existing in the plating bath.
Therefore, the plating bath is rapidly activated in a short period of time; that is, it loses its reducing power, whereby the life of the plating bath is extremely shortened.
The life thereof is approximately 60 minutes at most, allowing the plating bath to be used for only one plating.
For example, in Japanese Laid-open Patent Publication No. 60376 of 1996, a method in that influence of dissolved oxygen is lowered as much as possible by adding antioxidant or by supplying inert gasses to the plating bath is disclosed, however, even by employing this method, the life of the plating bath cannot be remarkably lengthened, and therefore, the plating bath can still be used for only one plating.
Therefore, the plating method cannot be prepared and stored in advance, so that a problem occurs in that the required amount of plating bath is prepared immediately before each plating, and therefore, operation efficiency is extremely poor.
Moreover, since a regenerating method of a plating bath which lost activity has not been known thus far, the plating bath has been disposed after being used only once, whereby waste has been great.
Also, problems occur in waste bath disposal.
Therefore, while the redox system electroless plating method has various advantages as mentioned above it has not been used widely in industry.
The main object of the invention is to provide a new plating method, enabling the industrial and wide use of the redox system electroless plating method having excellent characteristics as mentioned above.
Another object of the invention is to provide a new plating bath precursor that can be preferably used for the abovementioned plating method.
In order to solve the above problems, the present inventors variously examined the regenerating method of the plating bath to be used for the redox system electroless plating method.
As a result, by supplying an electrical current to the plating bath, when the metal ions of the redox system of the plating bath were reduced from a higher oxidation state to a low oxidation state, the bath was regenerated, whereby the bath was activated, enabling its use for plating.
It was also found that, when this activation process was combined with the plating process, the plating bath could be repeatedly used without limitation at an optional point in time after being prepared provided that the metal ions existed in the bath to form a plated layer, whereby the present invention was completed.
That is, the plating method of the invention is characterized in that, by oxidizing first metal ions of a redox system of a plating bath from a lower oxidation state to a high oxidation state, and second metal ions of said redox system are reduced and deposited onto the surface of an object to be plated, wherein a process is provided in which by supplying the electrical current to the plating bath, the first metal ions are reduced from said higher oxidation state to said lower oxidation state to thereby activate the plating bath.
The inventors also examined a plating bath storing method.
As a result, they found a method whereby the plating bath could be stored in the form of a so-called plating bath precursor which does not function as the plating bath by itself, that is, which was stable without the occurrence of reduction and deposition of the second metal ions.
In other words, even when this plating bath precursor is stored for a long period of time, since the second metal ions contained in the bath are prevented from being reduced and deposited freely during the period, the bath is regenerated as necessary by only reducing the first metal ions from a higher oxidation state to a lower oxidation state by supplying the electrical current, whereby the bath is activated, enabling it to be plated and used as a plating bath.
Therefore, the plating bath precursor of the invention comprises the first and second metal ions, and is made stable without reducing and depositing the second metal ions.
Also, at the xe2x80x9c99th Conference of the Surface Technology Societyxe2x80x9d held recently, a presentation was made wherein a redox system electroless silver plating bath using Co2+ as a reducing agent was added with a reducing agent whose reducing action was mild to selectively reduce oxidized cobalt ions (CO3+) in the bath (page 54 of the xe2x80x9cSummary report of 99th Conferencexe2x80x9d, by Junichi Kawasaki, Ken Kobayashi, and Hideo Honma, Surface Technology Society 1999).
That is, the reported result was that, since an oxidation and reduction potential of sodium sulfite as a reducing agent was between that of cobalt ions equivalent to the first metal ions and that of the silver ions equivalent to the second metal ions, without reduction and deposition of silver ions (Ag+) in the same bath, there was a possibility that only the oxidized cobalt ions (Co3+) existing in the bath could be selectively reduced into active cobalt ions (Co2+).
However, according to the examination by the inventors, this method has the following problems, and therefore, practical use of this method at an industrial level is considered difficult:
The reducing agent having the proper oxidation and reduction potential as mentioned above does not always exist in various combinations of the first and second metals, therefore this method cannot be applied to such combinations without the existence of the reducing agent.
Depending on the kind of reducing agent, co-deposits may occur as in the abovementioned prior-art conventional electroless plating.
If this method is repeated, and the plating bath is regenerated and used repeatedly, as in the case of the abovementioned prior-art conventional electroless plating, the reducing agent used for the reduction of metal ions accumulates in the bath as oxide, and therefore, the composition and concentration of the bath easily change, and the life of the plating bath is limited.
A report presented at the 99th Conference described an experiment using the abovementioned system. However, satisfactory results could not be obtained. In this presentation, the suggestion of the bath to be supplied with electrical current in place of a reducing agent was not mentioned.
On the other hand, according to the invention, as clearly understood from the results of the Examples described later, without the occurrence of the abovementioned various problems, excellent plating can be achieved. That is, as described later, if the electrical current density at the cathode when supplying an electrical current to the plating bath is adjusted, in various combinations of the first and second metals, the first metal ions can be satisfactorily reduced. Furthermore, since a reducing agent is not used, without the occurrence of problems concerning co-deposits and the bath life as mentioned above, a satisfactory plated layer can be formed.
Therefore, the contents of the abovementioned presentation do not suggest the present invention, but are only equivalent to the prior-art.